Title: WooCommerce returns raw HTML and "Missing user_id parameter" error during Klaviyo plugin connection

Question

Hi everyone,We are experiencing a critical issue when trying to connect the official Klaviyo plugin to WooCommerce. The authentication flow breaks down internally within our WordPress setup.The issue details:When clicking the "Connect Account" button, the site fails to redirect us to the standard, formatted WooCommerce page where we should approve the connection. Instead, the browser outputs raw HTML code (plain text).	If we inspect that raw HTML and manually extract the redirect link to force the connection, the URL fails within WordPress, throwing the following error:	"Missing user_id parameter"	Troubleshooting already done:We have completely deactivated our caching plugins (WP Fastest Cache) to rule out caching conflicts. The issue persists exactly the same way even with no cache active.It seems that WooCommerce is failing to process the REST API authentication request correctly, or the internal endpoint is returning an incorrect format (raw HTML) instead of executing the proper redirect. As a result, the vital user_id parameter is stripped or never appended to the query string.Has anyone encountered this raw HTML output during the API handshake, or does anyone know what server directive or WooCommerce configuration could be causing the user_id parameter to drop?Thank you in advance for your help!

Temi O. · Answer

Hi ​@Jstass,Thanks for sharing the detailed troubleshooting. From what I’ve found,this returned output and errorsuggests that the WordPress/WooCommerce authentication route is returning an unexpected response rather than completing the redirect.A few checks that may help:• Confirm WordPress permalinks are enabled and not set to Plain.• Confirm the site uses HTTPS with a valid SSL certificate.• Check for security, redirect, firewall, or other plugins affecting /wc-auth/ or /wp-json/ routes, not only caching plugins.• Confirm the Klaviyo WooCommerce plugin, WordPress, and WooCommerce are up to date.• Test the WooCommerce REST API directly and verify it returns JSON rather than HTML, a redirect, or a 401/403 response.If the standard WooCommerce API works but the Klaviyo authentication route still returns raw HTML, this will likely need Support Engineering review. Please contact Klaviyo Support with the store URL, plugin and platform versions, timestamp, response status, and a redacted screenshot or log. Please don’t share API keys or secrets publicly.Temi@Klaviyo

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